1. Field of the Invention
The present invention relates to a device for returning or holding a reciprocating mechanism, such as a valve opening-and-closing mechanism reciprocating as along a straight line or along a curved line, a swinging mechanism oscillating across its neutral position in a straight line or in a curved line or the like, to or at a predetermined position, and specifically to a return-to-neutral device which is applicable to a throttle-returnmechanism of an electronically-operated throttle valve.
2. Description of the Prior Art
In recent years, there have been proposed and developed various return-to-neutral devices suitable for a throttle-return mechanism for automotive engines. One such return-to-neutral device has been disclosed in an international application No. PCT/DE87/00404, filed Sep. 4, 1987 (PCT publication No. WO 88/02064, published Mar. 24, 1988), corresponding to Japanese patent publication No. 2-500677, published Mar. 8, 1990. Referring now to FIG. 3, there is shown a throttle-return device for an electronically-controlled throttle valve 2, as disclosed in the international application No. PCT/DE87/00404. As seen in FIG. 3, the throttle valve 2, interleaved in the intake-air passage 1, is driven by means of a throttle-valve actuator 3 such as an electric motor often called a "throttle motor". The throttle-valve actuator 3 is controlled in response to a control signal (or a drive signal) from an electronic control unit (ECU) or an electronic engine control module (ECM), irrespective of depression of the driver. The conventional throttle-return device includes a first resilient member 5 such as a helical torsion spring and a second resilient member 6 such as a helical torsion spring. The first resilient member 5 is connected to one end of the throttle-valve shaft 4 for biasing the throttle-valve shaft in a direction of wide-open throttle (clockwise, viewing FIG. 3), whereas the second resilient member 6 is connected to the other end of the throttle-valve shaft 4 for biasing the throttle-valve shaft in a direction of closed throttle. For example, when the ECU or the throttle-valve actuator (the throttle motor)does not operate owing to breakage in harness connector to the ECU, faulty throttle motor, loose or poor connector contact, or the like, the throttle-return device disclosed in the international application No. PCT/DE87/00404 functions to return or hold the electronically-controlled throttle valve 2 to a predetermined substantially neutral throttle-opening position or a predetermined intermediate angular position by virtue of proper balance between the clockwise biasing force caused by the first resilient member 5 and the counter-clockwise biasing force caused by the second resilient member 6. In the electronically-controlled throttle valve, for example due to a faulty throttle control section of the ECM or a faulty throttle motor, the electronically-controlled throttle valve 2 remains in a closed position, there is a tendency that the engine fails to start. In this case, the use of the throttle-return device for the electronically-controlled throttle valve 2 is effective to prevent the engine from failing to start. In automotive vehicles with an electronically-controlled throttle valve as described above, in order to compensate for load on/off modes at idle, the opening of the throttle valve 2 is controllable depending on engine load changes resulting from an increased load of a power steering, an air conditioner turned on or off, or changes in the other electrical loads. Thus, on vehicles employing such an electronically-controlled throttle valve, a so-called auxiliary-air control valve (generally abbreviated to "ACC valve"), or an idle speed control valve (generally abbreviated to "ISC"), or a fast idle control device (generally abbreviated to "FICD") can be eliminated. However, without the auxiliary-air control valve, there is no air-supply bypassing the throttle valve when starting the engine. Thus, if the electronically-controlled throttle valve 2 is placed into the closed position when the engine is stopped, intake air cannot be delivered into the engine cylinder with the throttle valve closed. In this case, the accelerator pedal must be depressed for starting the engine. To avoid this, automotive vehicles with an electronically-controlled throttle valve generally employ the previously-noted throttle-return device. The conventional throttle-return device disclosed in the international application No. PCT/DE87/00404 utilizes a proper balance of biasing-force between the two resilient members 5 and 6 in order to return the throttle valve to the predetermined intermediate angular position. The previously-noted conventional throttle-return device comprising at least the first and second resilient members, namely two different torsion springs 5 and 6, suffers from the drawback that it is difficult to set pre-loads of the first and second springs 5 and 6 in such a manner as to conform a first spring rate, measured when beginning to open the throttle valve 2 from the predetermined intermediate angular position to a second spring rate, measured when beginning to close the throttle valve 2 from the predetermined intermediate angular position. This lowers the controllability of the throttle valve in both directions, namely a direction of the opening-throttle mode and a direction of the closing-throttle mode. A slight manufacturing error of spring rate of the first resilient member 5 and a slight manufacturing error of spring rate of the second resilient member 6 may cause an increased deviation of a spring-loaded position of the throttle valve 2 from the predetermined intermediate angular position. As a result of the use of two resilient members 5 and 6, the structure of the conventional throttle-return device is complicated, thus increasing the time required for assembling, and total production cost of the throttle-return device. The complicated structure of the device composed of the two different resilient members 5 and 6 requires an increased mounting space. The two springs 5 and 6 must be installed on the respective different positions. This is troublesome. As set forth above, a conventional reciprocating-mechanism return device such as a throttle-return device composed of at least two resilient members is insufficient from the view point of a controllability of a reciprocating member (for example a throttle valve) in both directions of reciprocating motion, an increased degree of freedom in designing, a simple structure and reduction in installation efficiency and production costs.